To raise satellite acquisition and tracking performance, it is a main trend that most of the modernized GNSS will utilize a pilot signal as aid. That is, in addition to a data signal carrying navigation message, each satellite in the GNSS further transmits a pilot signal. Such modernized GNSS include new generation GPS (Global Positioning System) (L1C, L2C, L5 bands), Galileo (L1F (also referred to as E1), E5ab, E6C bands) and Compass Satellite System.
Taking Galileo L1F as an example, each satellite transmits two kinds of signals, data signal and pilot signal. As mentioned, the data signal carries navigation message. In contrast, the pilot signal is “dateless.” Both of the data signal and pilot signal are respectively modulated with different ranging code, that is, different PRN codes. In addition to PRN code, which is also referred to as a primary ranging code, the pilot signal is further modulated with a known secondary code. The data signal is modulated by 250 sps (symbol per second) data symbols. That is, the primary ranging code period is 4 ms. A data symbol is transmitted every 4 ms. The data symbol is usually unknown. The pilot signal also has the same primary ranging code period of 4 ms. The secondary code is of 25 chips. The pilot secondary code sequence is known. Each secondary code chip is referred to a pilot symbol here. The secondary code period is 4×25=100 (ms). That is, the secondary code transits once per 100 ms. Since the pilot signal is known, the integration interval can be greatly extended to a very long period, such as several seconds, for example.
A modern GNSS receiver, which has a receiver processor for carrying out navigation by using correlation result from a correlator of the receiver, may need to acquire/track data and/or pilot signal under different circumstances. That is, the receiver processor may require correlation result of the data or pilot signal only, or combination of both, depending on the application condition. For example, the receiver is to acquire only the pilot signal of a satellite with the whole workload of the correlator at a cold start state. After the pilot signal is acquired, the obtained information such as Doppler frequency, code phase and the like can be used to despread and demodulate the data signal of the same satellite. If there is enough aiding information, it is preferable for the correlator to acquire/track the pilot and data signals to increase SNR (Signal to Noise Ratio) and thus improve the performance. As described above, the pilot signal is dataless. Therefore, great SNR and long coherent integration time can be obtained by using pilot signal. Tracking the pilot signal can be used to detect troubles such as multipath interference, jamming and so on. To get navigation message, it is necessary to track and decode the data signal. If the signal strength is weak, it is preferred that combination of the data and pilot signal correlation results are used to reduce the effect of noises. In addition to the above conditions, there can be still various conditions in which different selections are required.
As described, there are various conditions for the correlator of the receiver. If the pilot signal and data signal are separately processed by different correlators, either the correlator processing the data signal or the correlator processing the pilot signal may often be idle. It will be a waste of hardware. Accordingly, it is a need that data correlation and pilot correlation share the same hardware resource. To share the correlator between the data and pilot signals, it is an important task to allocate the correlator more flexibly and efficiently.